The Britain-based Syrian Observatory for Human Rights says the air force is targeting opposition forces in the battle for the Mannagh air base outside of northern city of Aleppo. Rebels stormed the base near the border with Turkey on Sunday. They captured parts of it but could not hold on, because of the regime’s superior air power.

In neighboring Idlib province, there are heavy clashes outside several army bases near the government-controlled provincial capital. Government troops and fighter planes are hitting the rebels, the Observatory said.

Thursday’s fighting came a day after the announcement of a new U.S-Russian initiative to end two-year conflict through negotiations.